Bill Keller, a columnist at The New York Times and its former executive editor, will leave the paper to become editor in chief of The Marshall Project, a nonprofit journalism start-up focused on the American criminal justice system.

“It’s a chance to build something from scratch, which I’ve never done before,” Mr. Keller said, “and to use all the tools that digital technology offers journalists in terms of ways to investigate and to present on a subject that really matters personally.”